Network control system for controlling relative errors between network nodes

ABSTRACT

A network control system for controlling a plurality of nodes respectively corresponding to, and provided in, a plurality of radio zones. Each node has a corresponding base station. A mobil station travels among the plurality of radio zones and communicates with the base station of a respective node when travelling in a radio zone corresponding to the respective node. Each base station transmits a transmission signal to adjacent nodes and receives transmission signals transmitted from the base stations of adjacent nodes. Each base station includes (a) a reception automatic frequency control unit receiving the transmission signals transmitted by base stations of adjacent nodes, and performing an automatic frequency control operation for each received transmission signal, (b) a frequency error detection unit detecting a frequency error between the automatic frequency controlled transmission signals and a frequency obtained by adding a nominal frequency gap between base stations to the transmission frequency of the transmission signal of the node corresponding to the frequency error detection unit, (c) an averaging unit spatially filtering the frequency errors to produce a spatial mean value, and (d) a transmission frequency control unit controlling the transmission frequency of the node corresponding to the transmission frequency control unit in accordance with the spatial mean value, to substantially eliminate the frequency errors.

This application is a division of application No. 08/400,198, filed Mar.3, 1995, now pending, which is a File-Wrapper Continuation ofapplication No. 07/929,525, filed Aug. 14, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a network control system, andparticularly, to a network control system with a mobile station movingamong a plurality of cells and communicating with a base station in acell to which the mobile station belongs.

A mobile communication network divides a service area into a pluralityof radio zones (cells) and provides each radio zone with a base radiostation (hereinafter also referred to as a "node" depending on thesituation). A network control system employed to establish communicationbetween the base radio station and the mobile station is usually basedon a TDMA, FDMA, or CDMA method. According to these methods, the mobilestation synchronizes itself with the base radio station of a radio zonein which the mobile station is present and communicates with the basestation.

Each of the base radio stations has its own reference clock, andaccording to which, sends a signal. When travelling among the radiozones, the mobile station must synchronize itself with the frequency andtiming of a signal transmitted from the base radio station in a new zoneany time the mobile station enters the zone. Until synchronization isestablished, communication is suspended. It is necessary, therefore, toprovide means for eliminating relative errors or differences incontrolled values such as the phases and frequencies of clocks amongadjacent base radio stations.

2. Description of the Related Art

In a conventional network control system, a plurality of base radiostations serving as nodes are monitored by a central radio controlstation. These central radio control stations constitute a centralizedcontrol network for harmonizing and correcting the frequencies, andtiming of signals used to communicate among the base radio stations.With this conventional network, a mobile station is not required tosynchronize itself with the signals provided by the base radio stationswhen travelling among their zones.

This conventional network system determines the size of each radio zonedepending on the efficiency of frequency use, transmission power of themobile station, etc., and each zone is made relatively large in size(for example, several kilometers in radius).

In recent years, light and compact mobile stations have been developedhaving low transmission power. Recently, however, it has been requiredto improve the efficiency of frequency use, which has extremely reducedthe size of each radio zone to, for example, 50 to 100 meters in radius.A network involving such miniature zones must have a large number ofconventional base radio stations, thereby increasing the load on anupper apparatus such as a central radio control station and requiring acomplicated control system.

To solve these problems, an object of the present invention is toprovide a network control system for controlling a plurality of radiozones with a mobile station moving among the zones and communicatingwith a base radio station of one of the zones where the mobile stationis present, wherein each base radio station is capable of adjusting acontrolled value of its own transmission signal without relying on acentralized control network involving a central control station, therebyreducing the load on the central control station. In more detail, anobject of the present invention is to provide such a network controlsystem as above wherein even when the mobile station enters one radiozone from another radio zone, instant disabling of communication doesnot occur.

SUMMARY OF THE INVENTION

To attain the above objects, there is provided, according to the presentinvention, a network control system for eliminating a relative erroramong control signals transmitted from a plurality of nodes provided ina plurality of radio zones respectively, in which a mobile station ismoving among the radio zones. Each of the nodes comprises: a relativeerror detecting unit for detecting a relative error between anafter-controlled transmitting control signal to be transmitted toadjacent nodes and a received control signal from an adjacent node; anda control unit operatively connected to the relative error detectingunit, for controlling a before-controlled transmitting control signal insuch a way that the relative error becomes zero so as to output theafter-controlled transmitting control signal.

In the above system, it is preferable that the relative error detectingunit detects a plurality of the relative errors between theafter-controlled transmitting control signal and a plurality of receivedcontrol signals from a plurality of adjacent nodes, and that the systemfurther comprises a filtering unit, operatively connected between therelative error detecting unit and the control unit, for filtering noisefrom the relative errors.

Alternatively, the system comprises a filtering unit for filteringnoises from a plurality of received control signals from a plurality ofadjacent nodes to output a filtered signal, and the relative errordetecting unit is connected between the filtering unit and the controlunit so as to detect a relative error between the filtered signal andthe after-controlled transmitting control signal.

According to another aspect of the present invention, there is provideda network control system for controlling nodes involving fixed basestations and a mobile station with the same frequency for reception andtransmission as controlled values, each of the base stations comprising:a reception automatic frequency control unit for adjusting itself with atransmission frequency of an adjacent base station; a frequency errordetection unit for detecting a frequency error between an outputfrequency of the reception automatic frequency control unit and afrequency that is obtained by adding a nominal frequency gap betweenreference stations to the transmission frequency of its own node; anaveraging unit for spatially filtering the frequency errors of theadjacent base stations and providing a spatial mean value; and atransmission frequency control unit for controlling the transmissionfrequency of its own node in a way to eliminate the frequency errors.

According to still another aspect of the present invention, there isprovided a network control system for controlling a plurality of cellseach having a base radio station serving as a node for transmitting atransmission signal at optional timing that is a controlled value, eachbase radio station communicating with a mobile station when the mobilestation is located in the zone of the corresponding node, each of thebase radio stations comprising: a notification unit for notifyingadjacent base radio stations of transmission timing information of itsown node; and a correction unit for receiving, as input signals, piecesof transmission timing information of the adjacent base radio stations,and correcting at least the timing of the transmission signal of its ownnode in a way to minimize a difference between the timing of the signalof its own node and the timing of the transmission signals of theadjacent base radio stations that have been spatially filtered.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of the preferred embodimentswith reference to the accompanying drawings, wherein;

FIG. 1A and FIG. 1B are block diagrams showing principle constructionsof a node in a network control system according to the invention;

FIG. 2 and 3 are block diagrams both showing a system employing aspatial filter according to an aspect of the invention;

FIG. 4 and 5 are block diagrams both showing a system employing a timefilter according to another aspect of the invention;

FIG. 6 and FIG. 7 are block diagrams both showing a system employing aspatial filter and a time filter according to still another aspect ofthe invention;

FIG. 8 is a block diagram showing a system employing a spatial filterand a time filter according to still another aspect of the invention;

FIG. 9 is a block diagram showing a system employing a spatial filterand a time filter according to still another aspect of the invention;

FIG. 10 is a block diagram showing a system employing variable intervalsaccording to still another aspect of the invention;

FIG. 11 is a block diagram showing a system employing variable filteringcharacteristics according to still another aspect of the invention;

FIG. 12 is a block diagram showing a system employing variable levelconverting characteristics according to still another aspect of theinvention;

FIG. 13 is a block diagram showing a system for controlling frequenciesaccording to still another aspect of the invention;

FIG. 14 is a view showing an arrangement of base stations;

FIG. 15 is a block diagram showing a system for controlling timingaccording to still another aspect of the invention;

FIG. 16 is a schematic view showing a network according to theinvention;

FIG. 17 is a block diagram showing an embodiment based on the system ofFIG. 2;

FIG. 18 relates to FIG. 17 and shows a result of simulation ofcharacteristics of signal quantity to time according to a prior art;

FIG. 19 is a view showing a result of simulation of characteristics ofsignal quantity to time according to FIG. 17;

FIG. 20 is a block diagram showing an embodiment based on the system ofFIG. 4;

FIG. 21 is a view showing a result of simulation of a system with notime filter;

FIG. 22 is a view showing a result of simulation of characteristics ofFIG. 20;

FIG. 23 is a block diagram showing an embodiment based on the system ofFIG. 6;

FIG. 24 is a block diagram showing an embodiment based on the system ofFIG. 10;

FIG. 25 is a block diagram showing a phase detection unit according toan embodiment of the invention;

FIG. 26 is a timing chart showing operations of the phase detection unitof FIG. 25;

FIG. 27 is a block diagram showing an embodiment based on the system ofFIG. 11;

FIG. 28 is a block diagram showing an embodiment based on the system ofFIG. 12;

FIG. 29 is a block diagram showing an error detection unit according toan embodiment of the invention;

FIG. 30A to FIG. 30C are views showing a time averaging operationcarried out by an averaging unit according to an embodiment of theinvention;

FIG. 31 is a view showing a result of simulation of time changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation carried out in the averaging unitaccording to the invention;

FIG. 32 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofa transversal filter, carried out in the averaging unit according to theinvention;

FIG. 33 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofa complete integration filter, carried out in the averaging unitaccording to the invention;

FIG. 34 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofan incomplete integration filter, carried out in the averaging unitaccording to the invention;

FIG. 35 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation carried out in the averaging unitwith use of a reference station R set among base stations according tothe invention;

FIG. 36 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofa transversal filter, carried out in the averaging unit with use of thereference station R according to the invention;

FIG. 37 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofa complete integration filter, carried out in the averaging unit withuse of the reference station R according to the invention;

FIG. 38 is a view showing a result of simulation of temporal changes infrequency errors among adjacent base stations, obtained from a simplespatial arithmetic averaging operation and a time averaging operation ofan incomplete integration filter, carried out in the averaging unit withuse of the reference station R according to the invention;

FIG. 39 is a block diagram showing a first embodiment based on thesystem of FIG. 15;

FIG. 40 is a schematic view showing a network for explaining theembodiment of FIG. 39;

FIG. 41 is a block diagram showing a second embodiment based on thesystem of FIG. 15;

FIG. 42 is a schematic view showing a network for explaining theembodiment of FIG. 41;

FIG. 43 is a schematic view showing a network according to an embodimentof the invention;

FIGS. 44A to 44C are views explaining characteristics of the network ofFIG. 42;

FIGS. 45A and 45B are views explaining characteristics of the network ofFIG. 42 with a node being added to or removed from the network;

FIG. 46 is a schematic view showing a network according to anotherembodiment of the invention;

FIG. 47 is a schematic view showing network according to still anotherembodiment of the invention;

FIG. 48 is a schematic view showing a network according to still anotherembodiment of the invention;

FIG. 49 is a view showing a result of simulation of pull-incharacteristics (characteristics of time to signal quantity) with asingle uncontrollable node;

FIG. 50 is a view showing a result of simulation of pull-incharacteristics (characteristics of time to signal quantity) with alarge number of uncontrollable nodes;

FIG. 51 is a block diagram showing a node having a pull-in rangefunction according to an embodiment of the invention;

FIG. 52 is a view showing a result of simulation of pull-incharacteristics (characteristics of time to signal quantity) of theembodiment of FIG. 51;

FIG. 53 is a view showing a result of simulation of pull-incharacteristics (characteristics of time to signal quantity) withadjacent nodes being selected at random according to the invention;

FIG. 54 is a view explaining a rule of node selection according to theinvention;

FIG. 55 is a view showing a result of simulation of pull-incharacteristics (characteristics of time to signal quantity) withadjacent nodes being selected based on the rule according to theinvention;

FIG. 56 is a view showing a combination of frequency control anddiversity reception according to an embodiment of the invention;

FIG. 57 is a view showing a combination of timing control and diversityreception according to another embodiment of the invention; and

FIG. 58 is a schematic view showing a conventional network.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For better understanding the background of the present invention, aconventional network control system will first be described withreference to FIG. 58. In FIG. 58, 700 represents a plurality of baseradio stations, and 800 represents a central control station. Theconventional network control system is a centralized control network inwhich each base radio station 700 is synchronized with the centralcontrol station 700. The problem in the conventional system is that, inaccordance with the increase of the number of the base radio stations700, the load on the central radio control station is increased.

Embodiments of the present invention will be described in the following.

Basic concept and operation of an embodiment shown in FIG. 1A and FIG.1B

FIG. 1A shows a network control system according to an embodiment of thepresent invention.

In FIG. 1A, 1 is a relative error detecting unit for detecting arelative error between a transmission signal A(t+1) after transmission,i.e., an after-controlled transmitting control signal A(t+1) to betransmitted to adjacent nodes and a received control signal Ai(t) froman adjacent node; and 3 is a control unit having an output connected tothe relative error detecting unit, for controlling a before transmissionsignal (a before-controlled transmitting control signal) A(t) to outputthe after controlled in such a transmitting control signal A (b+1) waythat the relative error becomes zero.

The relative error detecting unit 1 detects a plurality of relativeerrors between the after-controlled transmitting control signal A(t+1)and a plurality of received control signals Ai(t) (i=1, 2, . . . , n)from a plurality of adjacent nodes. In FIG. 1A, the system furthercomprises a filtering unit 2, operatively connected between the relativeerror detecting unit 1 and the control unit 3, for filtering noises fromthe relative errors.

In the following, the before-controlled transmitting control signal A(t)is called a controlled value to be controlled in the node; theafter-controlled transmitting control signal A(t+1) is called a nextcontrolled value to be controlled in the next node; and the receivedcontrol signal Ai(t) is called a controlled value already controlled inan adjacent node. The controlled value is a synchronizing signal such asa clock timing or a frequency. The controlled value Ai(t) may bereferred to as a first signal; the controlled value A(t) may be referredto as a second signal; and the controlled value A(t+1) may be referredto as a third signal.

A node under consideration involves a controlled value A(t) at time tand controls the next controlled value A(t+1) for the next time t+1 asfollows:

    A(t+1)=A(t)+C(t)                                           (1)

where C(t) is a value obtained by a time filtering operation Ft carriedout at the time t and expressed as follows:

    C(t)=Ft{B(t), B(t-1), . . . , B(t-m)}                      (2)

where B(t) is a value obtained by a space filtering operation Fs carriedout at the time t and expressed as follows:

    B(t)=Fs{El(t), E2(t), . . . , En(t)}                       (3)

where Ei(t) (i=1, 2, . . . , n) are relative errors at the time tbetween the controlled value of the given node and controlled valuesAi(t) of nodes that are referred to by the given node and expressed asfollows:

    Ei(t)=A(t)-Ai(t) (i=1, 2, . . . , n)                       (4)

In this way, the node under consideration provides and receivesinformation of controlled values to and from the adjacent nodes. Withouta central control station, this method can remove relative errors ofcontrolled values between a given node and adjacent nodes, therebycorrectly controlling controlled values of the system as a whole.

The spatial filtering carried out on error signals removes or reducesthe influences of disturbance and abnormality occurring on somereference signals with respect to space. The time filtering removes orreduces the influences of disturbance and abnormality occurring on someof a string of continuous error signals with respect to time.

Order of the spatial filtering Fs and time filtering Ft may be reversed.It is possible to carry out at least one of the space and time filteringoperations.

Further, as shown in FIG. 1B, the space and time filtering may becarried out on the received controlled value Ai(t) before detecting therelative error.

Aspects of the embodiments of invention of FIGS. 2 to 9

FIGS. 2 to 9 show preferred aspects of the embodiments of the inventionbased on the system of FIG. 1A and FIG. 1B.

According to the embodiment shown in FIG. 2, a network control systeminvolves a plurality of base radio stations serving as nodes fortransmitting signals. A mobile station communicates with any one of thebase radio stations while the mobile station is present in the radiozone of the base radio station in question. Each of the nodes (baseradio stations) comprises a first operation unit 11, a first errordetection unit 12, and a control unit 13. The first operation unit 11spatially filters reference signals (which are the controlled values inFIG. 1A or 1B) received from adjacent nodes. The first error detectionunit 12 provides an error signal indicating a difference between anoutput signal of the first operation unit 11 and an output signal A(t+1)of its own node. The control unit 13 controls a synchronization objectinput signal A(t) in a way to minimize the error signal, and providesthe adjacent nodes with a reference signal A(t+1) of its own node.

In FIG. 3, the first error detection unit 12 in FIG. 2 is replaced witha plurality of second error detection units 12_(l) to 12_(k) eachproviding an error signal indicating a difference between acorresponding reference signal Ai(t) (i=1, 2, . . . , n) and an outputsignal A(t+1) of its own node. The first operation unit 11 in FIG. 2 isreplaced with a second operation unit 14 for spatially filtering outputsignals from the second error detection units 12_(l) to 12_(k) andproviding a control unit 13 with a control signal.

In FIG. 4, a first storage unit 15 stores a reference signal Ai(t) froman adjacent node at a plurality of instants. A third operation unit 16filters, with respect to time, at least the reference signals of aplurality of instants provided by the first storage unit 15 and providesa first error detection unit 12 with a filtered signal filtered withrespect to time. Other arrangements in FIG. 4 are the same as those inFIG. 2.

In FIG. 5, a third error detection unit 17 prepares an error signalindicating a difference between an output signal A(t+1) of its own nodeand a reference signal Ai(t). A second storage unit 18 stores the errorsignal at a plurality of instants. A fourth operation unit 19 filters,with respect to time, at least the error signal of a plurality ofinstants provided by the second storage unit 18 and provides a controlunit 13 with the signal, filtered with respect to time, as a controlsignal. Other arrangements of FIG. 5 are the same as those of FIG. 3.

In addition to the arrangement of FIG. 2, an arrangement of FIG. 6 has athird storage unit 21 and a fifth operation unit 22. The third storageunit 21 stores an output signal of the first operation unit 11 at aplurality of instants. The fifth operation unit 22 filters, with respectto time, at least an output signal of plurality of instants provided bythe third storage unit 21 and provides a first error detection unit 12with the signal filtered with respect to time.

In addition to the arrangement of FIG. 2, an arrangement of FIG. 7 has aplurality of first storage units 15_(l) to 15_(k), a plurality of thirdoperation units 16_(l) to 16_(k), and a sixth operation unit 24 forspatially filtering temporally filtered reference signals provided bythe third operation units 16_(l) to 16_(k). The term spatially filteringrefers to a filtering with respect to space, and the term temporallyfiltering refers to a filtering with respect to time.

In addition to the arrangement of FIG. 3, an arrangement of FIG. 8 has afourth storage unit 26 and a seventh operation unit 27. An error signalspatially filtered by the second operation unit 14 is temporallyfiltered by the fourth storage unit 26 and seventh operation unit 27 andprovided as a control signal to a control unit 13.

FIG. 9 shows a combination of the arrangements of a modification of FIG.1B, wherein the reference signals Ai(t) illustrated as Al(t) . . .An(t), are spatially filtered by the first operation unit 11. The firsterror detection unit 12 prepares an error signal indicating a differencebetween an output signal A(t+1) of its own node and the output signal ofthe first operation unit 11. The second storage unit 18 stores the errorsignal at a plurality of instants. The forth operation unit 19temporally filters at least the error signal of a plurality of instantsprovided by the second storage unit 18 and provides the control unit 13with the temporally filtered signal as a control signal.

Aspects of the embodiments of the invention of FIGS. 10 to 12

FIGS. 10 to 12 show preferred examples of apparatuses directlyemployable for the basic system of FIG. 1A.

Each node usually periodically updates the timing and parameters (whichare the controlled values or the controlled transmitting signals inFIGS. 1A and 1B) of its own signal in order to synchronize thecontrolled values with those of adjacent nodes. Shortening signalupdating intervals will reduce a convergent time of the system. This,however, raises a problem that the system is easily affected anddestabilized by disturbance. Elongating the time width of a temporalfiltering operation will improve an effect of suppressing the influenceof disturbance. This, however, raises a problem of extending aconvergent time of the system. Improving the accuracy of quantizationwill minimize a residual error, to thereby achieving accuratesynchronous control. This, however, causes a problem of elongating aconvergent time of the system.

In FIG. 10 of the invention, a detection unit 2-1 detects a differentialsignal E between an output signal Q (corresponding to the controlledvalue of FIG. 1A) of its own node and a signal R from another node. Asignal generation unit 2--2 controls the signal Q in a way to minimizethe differential signal E according to a timing signal T from a controlunit 2-3. The control unit 2-3 changes intervals of activating thesignal generation unit 2--2 when the system is initialized or dependingon the magnitude of the differential signal E. When the system isinitialized according to, for example, a power ON reset signal, or whenthe differential signal E is greater than a predetermined value T_(H),the control unit 2-3 shortens the intervals of activation to promoteconvergence to the signal R, and in other cases, elongates the intervalsof activation to avoid an influence of disturbance.

In FIG. 11, a detection unit 2-1 detects a differential signal E betweena signal Q of its own node and a signal R from another node. A signalgeneration unit 2--2 controls the signal Q in a way to minimize thedifferential signal E. A filter 2-4 spatially or temporally filters thesignal R provided by another node, the differential signal E provided bythe detection unit 2-1, or a control signal C prepared for the signalgeneration unit 2--2 from the differential signal E. A control unit 2-5changes the filtering characteristics of the filter 2-4 when the systemis initialized or depending on the magnitude of the differential signalE. When the system is initialized or when the differential signal E isgreater than a predetermined value T_(H), response of the filter 2-4 isquickened to speed up convergence to the signal R. In other cases, theresponse is slowed down to avoid disturbance.

In FIG. 12, a detection unit 2-1 detects a differential signal E betweena signal Q of its own node and a signal R of another node. A signalgeneration unit 2--2 controls the signal Q in a way to minimize thedifferential signal E. An input/output level conversion unit 2-6converts input and output levels of the signal R from another node, ofthe differential signal E, or of a control signal C prepared for thesignal generation unit 2--2 from the differential signal E. A controlunit 2-7 changes the level converting characteristics of theinput/output level conversion unit 2-6 when the system is initialized ordepending on the magnitude of the differential signal E. When the systemis initialized or when the differential signal E is greater than apredetermined value T_(H), the level conversion characteristics are madeto be coarse for quantization or linearization, to expedite convergenceto the signal R. In other cases, quantization accuracy around an inputlevel of 0 is improved to increase the accuracy of convergence.

Aspect of the embodiments of the invention of FIG. 13

FIG. 13 shows a preferred embodiment of the invention for controllingfrequencies as controlled values. This embodiment is directly applicablyfor the basic system of FIG. 1.

Generally, radio channel frequencies f_(A) and f_(B) which are thecontrolled values of FIG. 1A) of base stations of adjacent nodes have anominal frequency gap f_(S) to minimize an interference between thefrequencies. When a mobile station travels among zones from one basestation to an adjacent base station, the mobile station changes itsfrequency from f_(A) to f_(B) for communicating with a correspondingbase station.

The oscillation frequency of a local oscillator that determines a radiochannel frequency has a finite stability. For this reason, a frequencyallocated for one base station always involves a finite error. In aconventional mobile radio communication system, this frequency error isabsorbed by an automatic frequency control (hereinafter referred to asAFC) circuit provided for the mobile station.

Accordingly, when the mobile station travels from one zone to another(hereinafter, the travelling of the mobile station among zones or cellswill be referred to as a hand-over), the mobile station receivesinformation related to the radio channel frequency f_(B) of the basestation of the zone into which the mobile station has entered. Even ifsuch information is provided, the mobile station must absorb, throughthe AFC circuit, an error frequency (f_(B) -f_(B) ') between a nominalfrequency f_(B) and an actual frequency f_(B) ' of the base station inquestion. In practice, a signal provided by a base station with whichthe mobile station was communicating before a hand-over involves afrequency error, so that a maximum frequency error that must be absorbedmay be twice the frequency error allowed for the base station.

Accordingly, the AFC circuit extensively operates during a transientperiod for every hand-over, and until the operation of the AFC circuitis completed, a call or data transmission is suspended thereby causinginconvenience for a user. It is pointed out that effective use offrequencies is achievable by reducing the size of each zone (cell) andby spatially repeatedly using the frequencies. This causes manyhand-overs to occur, thereby causing many momentary stoppages ofcommunication, and deteriorating system serviceability.

To solve this problem, the invention intends to eliminate relativefrequency errors between adjacent base stations where hand-overs occur,this requires a mobile station to only change its frequency for anominal radio channel frequency gap f_(S) whenever a hand-over occurs.

To achieve this, each base station detects a difference between thesignal frequency of an adjacent base station and the signal frequency ofits own node and zeroes the difference by controlling the transmissionfrequency of its own node.

In the preferred network control system of FIG. 13, a receptionautomatic frequency control unit 3--3 of a base station 3-1 carries outan AFC operation according to the transmission frequency of an adjacentbase station. A frequency error detection unit 3-4 compares a frequencyf_(R) provided by the control unit 3--3 with a frequency "f_(A) +f_(S) "obtained by adding a prescribed frequency gap f_(S) between basestations to a transmission frequency f_(A) of the base station 3-1, andprovides a frequency error Δf.

Such frequency error Δf is detected between the base station in questionand every adjacent base station. An averaging unit 3-5 averages thedetected errors by spatially filtering the errors as shown in FIG. 1.

When an "i"th base station has a nominal frequency f_(i) and an actualfrequency f_(i) '(t) at time t, a relative frequency error Δf_(i) (t)with respect to any adjacent base station j (j=1, 2, . . . , N) withwhich a hand-over may occur will be expressed as follows: ##EQU1## wheref_(sij) is expressed as follows:

    f.sub.sij =f.sub.j -f.sub.i                                (6)

Namely, f_(sij) is a difference between the nominal frequencies of thebase stations i and j and is an integer multiple of the nominalfrequency gap f_(S). An accumulation of _(j=1) Σ^(N) is carried out forj=1, 2, . . . , N.

The averaging unit 3-5 provides a transmission frequency control unit3-6 with the error information Δf_(i) (t), and the control unit 3-6controls a transmission frequency f_(A) of its own station at time t+1as follows:

    f.sub.i (t+1)=f.sub.i (t)-αΔf.sub.i (t)        (7)

where α is a numerical coefficient of 0<α≦1.

When averaging relative frequency errors between a given base stationand adjacent base stations, it is possible to use a weight of C/N ofeach signal to calculate a weighted mean, or exclude extremely largeerrors. These averaging techniques may improve the stability of thecontrol system.

The averaging unit 3-5, according to the invention, can also average thefrequency errors Δf_(i) (t) by temporally filtering the frequencyerrors. This may eliminate adverse effects caused by noise, etc., incommunication paths and smooth momentary large errors, thereby furtherstabilizing the system. This is particularly needed when the C/N of areceived signal is low.

In this way, relative frequency errors between a given base station andadjacent base stations are eliminated. This effect, however, is onlylocal. When an absolute frequency value must be secured, the aboverelative frequency control technique carried out between adjacent basestations is insufficient because it never guarantees an absolutefrequency value.

FIG. 14 shows a method of globally eliminating frequency errors from acommunication system, according to the invention. In the figure, one ofbase stations 3-1 is selected as a reference station R that transmits anabsolute transmission frequency. Being pulled by this absolutetransmission frequency, each of the other base stations absorbs thefrequency error Δf mentioned above, thereby guaranteeing an absolutefrequency for the whole system and improving reliability of the system.

This invention is applicable not only for a communication systememploying the same frequency for transmission and reception but also fora communication system employing different frequencies for transmissionand reception with each base station and mobile station alwayscontrolling a frequency difference between a reception frequency and atransmission frequency.

Aspect of the embodiment of the invention of FIG. 15

FIG. 15 shows an aspect of the invention based on the system of FIG. 1Aor 1B. The controlled value of this aspect is timing.

A network control system of this aspect involves base radio stations 101serving as nodes. Each of the base radio stations 101 comprises anotification unit 103 for notifying adjacent base radio stations 101 oftransmission timing of its own node, and a correction unit 104 forreceiving transmission timing information as an input signal andcorrecting at least the phase of a transmission signal of its own nodein a way to minimize a difference between the timing of its own node andthe timing of a transmission signal that has been received from anadjacent base station 101 and spatially filtered.

The base radio stations 101 communicate individual transmission timinginformation with one another through the notification unit 103. Thecorrection unit 104 of each of the base radio stations 101 corrects,according to the transmission timing information, at least the phase ofthe transmission signal of its own node in a way to minimize adifference between the transmission timing of its own node and those ofsignals of the adjacent base stations. As a result, the phase differencebecomes zero in the end. This embodiment forms a distributed controlnetwork system with the base radio stations 101 serving as nodes,similar to the model of FIG. 16.

The correction mentioned above will be explained in more detail. Thelevel (corresponding to a frame phase and clock frequency in the TDMAmethod, and a clock phase and clock frequency in the FDMA or CDMAmethod) of a given node (base radio station 101) of the distributedcontrol network is expressed as B(i, t), where i is a node number and tis time. Each node compares the level with those of adjacent nodes andcorrects a difference between them.

With node numbers l to N (for the sake of convenience, i>N) to becompared with one another, the difference of level between a given nodeand adjacent nodes after a spatial filtering operation is expressed asfollows:

    ΔB(i, t-1)=B(i, t-1)-.sup.N Σ.sub.n=1 B(n, t-1)/N (8)

Accordingly, the level B(i, t) of the node i is corrected as follows:

    B(i, t)=B(i, t-1)- αΔB(i, t-1)!.sub.AVE        (9)

where α is a coefficient (0<α≦1) and !AVE is averaging.

For any of the TDMA, FDMA, and CDMA methods, the invention equalizes thelevels of all nodes in the end and synchronizes all base radio stationswith one another.

According to the invention, each node (base radio station 101) isconnected to adjacent nodes, so that, even if any one of the adjacentnodes fails, the remaining nodes will not be affected by the failurebecause they can control themselves according to timing informationprovided by the normal adjacent nodes. Even when the number of nodes(base radio stations 101) is increased, an increase in the number ofadjacent nodes around a given node will be small so that load on thenode will be substantially unchanged.

According to the invention, each node may have a pull-in range to onlycontrol controlled values that exist in the pull-in range. Thistechnique excludes uncontrollable values from mutual synchronouscontrol, thereby reducing a destabilized period of the system.

According to the invention, base stations that provide controlled valuesmay be selected according to a certain rule. This technique prevents afluctuation in the controlled values from spreading to adjacent nodes atrandom, thereby reducing a destabilized period of the network.

According to the invention, the spatial filtering operation mentionedabove may provide a weighted mean of relative errors of controlledvalues according to a reception electric field strength selected bydiversity reception, to omit a special S/N detection circuit.

FIG. 17 shows a more practical embodiment of the network control systemof FIG. 2. In the figure, the same parts as those of FIG. 2 arerepresented with like numerals and their explanations will not berepeated. In FIG. 17, a read only memory (ROM) 11a receives, throughaddress lines, 4-bit reference signals (controlled values such asfrequency signals, timing signals, and reception power signals) fromfour adjacent nodes. The ROM 11a stores a table for providing an averageof the address inputs of 4 bits ×4 through four bits of an 8-bit dataline.

A comparator 12a is, for example, a 4-bit digital subtracter. Thiscomparator forms an error detection unit 12. The comparator 12a, whichis the 4-bit digital subtracter, subtracts a 4-bit input value fromanother 4-bit input value and provides a 4-bit result.

A coefficient multiplier 13a, an adder 13b, and a controller 13c form acontrol unit 13. The coefficient multiplier 13a is, for example, a 4-bitdigital multiplier, which multiplies a 4-bit input value by a 4-bitcoefficient α (0<α<1) and provides a 4-bit result.

The adder 13b is, for example, a 4-bit digital adder, which receives a4-bit digital input, i.e., a synchronization object signal (a controlledvalue such as clock information and transmission power information) ofits own node as well as a 4-bit output of the coefficient multiplier13a, adds them to each other, and provides a 4-bit result. This adder13b provides an initial value or an offset. If such initial value oroffset is not needed, the adder 13b may be omitted.

The controller 13c comprises, for example, a 4-bit digital accumulator(ACC), which adds a 4-bit input value to a stored last output value andprovides a 4-bit output. The output of the controller 13c is provided tothe comparator 12a, used as a synchronization object signal for its ownnode, and transmitted as a reference signal to adjacent nodes throughoutput means.

Operations of this embodiment will be explained. The ROM 11a receivesreference signals from four adjacent nodes and provides a mean value ofthe four reference signals. The comparator 12a receives the mean valueand subtracts it from an output value of the controller 13c. When agiven node i and an adjacent node j have signal quantities Si(n) andSj(n), respectively, at time n, an output error signal of the comparator12a of the given node is expressed as follows:

    ΔSi(n)={.sub.j=1 Σ.sup.N Sj(n)}/N-Si(n)        (10)

where N is the total number of the input reference signals, which isequal to four in this embodiment. The first term of the right side ofthis equation indicates an output mean value of the ROM 11a.

The output error signal ΔSi(n) from the comparator 12a is multiplied bythe coefficient α in the coefficient multiplier 13a, added to asynchronization object input signal A in the adder 13b, and added to thelast signal in the controller 13c. As a result, the controller (ACC) 13cprovides a signal Si(n+1), which is expressed as follows:

    Si(n+1)=Si(n)+α·ΔSi(n) (n≧)

    Si(n+1)=Si(n)+α·ΔSi(n)+A(n=0)         (11)

Under a steady state, the signal value of the given node agrees with themean value of the reference signals, so that ΔSi(n)=0. Namely, thesignal of the given node is unchanged. If the signal value of the givennode is changed by some reason, or if the reference signals are changed,the signal value Si(n+1) is controlled to zero the ΔSi(n). In this way,each node synchronizes itself with adjacent nodes, and all nodes in thenetwork are synchronized with one another.

If one of the four reference signals is abnormally large or small, theaveraging operation, i.e., the spatial filtering operation of the ROM11a, reduces the influence of the abnormality to one fourth so that thesignal quantity to be synchronized as a controlled value may bestabilized.

The spatial filtering operation by the ROM 11a is not limited to thesimple averaging operation. For example, a mean value may be calculatedafter excluding values that are greater (or smaller) than a thresholdvalue.

FIGS. 18 and 19 show results of simulations of the first embodiment ofFIG. 17. FIG. 18 shows a result of simulation of characteristics ofsignal quantity Si(n) versus time with a simple averaging operation suchas one shown in the embodiment being used as a spatial filteringoperation. FIG. 19 shows characteristics of signal quantity versus timewith a spatial filtering operation being carried out by averaging valuesafter excluding those exceeding a threshold. In each figure, changes insignal quantity of all nodes (36 nodes) are plotted on the same graph.

In FIG. 18, if one reference signal changes to an abnormally great valueat a certain time point, as indicated by "a", then a signal quantity asa synchronization object of each node gradually converges to a certainvalue. In FIG. 19, if one reference signal exceeds the threshold and isfixed at an abnormally great value, as indicated by "a," this referencesignal is excluded. As a result, this abnormality does not influence theother nodes.

The spatial filtering operation may be achieved by obtaining mostfrequent input values (medians), or by averaging values after excludingmaximum and minimum values. These techniques also remove the influencesof extreme values.

The above embodiment compares a spatially filtered result of inputreference signals with a signal of its own node. It is also possible, asshown in FIG. 3, to compare input reference signals with a signalquantity of its own node to provide a plurality of error signals, andthen spatially filter the error signals to provide the control unit 13with a result of the spatial filtering.

In this case, the following spatially filtered result ΔSi(n) is obtainedaccording to the equation (10):

    ΔSi(n)={.sub.j=1 Σ.sup.N ΔSij(n)}/N      (12)

    ΔSij(n)=Sj(n)-Si(n)                                  (13)

FIG. 20 is a schematic view showing an embodiment of the network controlsystem of FIG. 4 according to the invention. In the figure, the sameparts as those of FIGS. 4 and 17 are represented with like numerals andtheir explanations will not be repeated. In FIG. 20, a 3-stage shiftregister 15a of 4-bit width forms a first storage unit 15. The shiftregister receives a 4-bit reference signal, shifts the signal to theright at each clock pulse, and stores the reference signal for the lastthree clock periods.

A ROM 16a forms a third operation unit 16. The ROM 16a stores a table,which provides a mean value of address inputs of 4 bits ×4 through fourbits of an 8-bit data line.

According to this embodiment, the shift register 15a shifts a referencesignal provided by an adjacent node to the right at every clock pulse,and the reference signal is applied to a 4-bit address terminal of theROM 16a. The other three 4-bit address terminals of the ROM 16a receivethe reference signal sampled at the last three time points (the first,second, and third latest clock pulses) from the shift register 15a inparallel. The ROM 16a calculates a mean value (a moving average value)of the reference signal sampled at the four consecutive instants fromthe past to the present, and provides a comparator 12a with a result ofthe averaging. In this way, the ROM 16a temporally filters the referencesignal and provides the comparator 12a with a result of the temporalfiltering. The comparator 12a compares the result with an output signalSi(n) of a controller 13c and provides an error signal ΔSi(n) expressedas follows:

    ΔSi(n)={.sub.m-1 Σ.sup.M Sj(n-m+1)}/M-Si(n)    (14)

where M is a period (the number of instants) for which the movingaverage is calculated. In this embodiment, M is 4.

The error signal ΔSi(n) at time n is supplied as a control signal to acontrol unit 13 having the same arrangement as the control unit 13 ofFIG. 17. The control unit 13 provides a signal Si(n+1) for the next timen+1 as follows:

    Si(n+1)=Si(n)+αΔSi(n)+A                        (15)

This output signal is supplied to the comparator 12a, used as, forexample, a clock for its own node, and transmitted as a reference signalto adjacent nodes. In the above equation (15), A is a synchronizationobject input signal.

Under a steady state of this embodiment, the signal value of a givennode agrees with a moving average of reference signals of the node. Inthis case, ΔSi(n)=0, so that the signal of the given node is unchanged.If the signal value of the given node is changed due to some reason, orif the reference signals are changed, the signal value Si(n+1) of thegiven node is controlled to zero the error signal ΔSi(n). In this way,each node operates to synchronize itself with adjacent nodes, so thatall nodes of the network synchronize with one another.

If reference signal values become abnormally large or small, the movingaverage operation suppresses the influence of the abnormality to onefourth, so that the signal quantity of a given node may graduallychange.

This will be explained in more detail with reference to FIGS. 21 and 22involving 36 nodes and 3 reference signals. FIG. 21 showscharacteristics of signal quantity versus time without temporalfiltering (without the shift register 15a in the embodiment of FIG. 12).FIG. 22 shows characteristics of signal versus time with the arrangementof FIG. 20. Similar to FIGS. 18 and 19, each of FIGS. 21 and 22 plotschanges in signal quantity of all nodes on the same graph.

In FIG. 21, the value of one reference signal abnormally increases asindicated by "d." Then, signal quantities of the other nodes slightlybut steeply change in magnitude as indicated by "e" and then graduallyconverge to a fixed value. In FIG. 22, the signal quantities of theother nodes slightly and gradually change as indicated by "g" when thevalue of one reference signal abnormally increases as indicated by "f."In this way, the invention can achieve stable network synchronizationcontrol that is not affected by abnormal changes and disturbance inreference signals.

The above embodiment achieves a temporal filtering operation through asimple moving average. It is also possible to employ an intervalaveraging for a certain period, a weighted mean with a weight as a valuecorresponding to a time counted from the present moment, most frequentinput values (medians), or averaging after excluding maximum and minimumvalues. When the weighted mean technique with a weight corresponding toa time counted from the present moment is employed, an inverse number ofthe time from the present moment, for example, is used as a weight. Inthis case, the older the value, the lesser the value influences apresent controlled value, so that it may shorten a convergence time froman initial state to a steady (stabilized) state compared with using thesimple moving average technique. When the medians or the averaging afterexcluding maximum and minimum values is used, sudden disturbance can beremoved.

The above embodiment compares, in a given node, a temporally filteredresult of input reference signal values stored at several time pointsfrom the present moment to the past with a signal of the node itself. Asshown in FIG. 5, signal values stored at several instants may becompared with a signal quantity of the given node to provide a pluralityof error signals, which are temporally filtered to provide a controlinput.

In this case, a result of the temporal filtering is expressed as followsaccording to the equation (14):

    ΔSi(n)={.sub.m=1 Σ.sup.M ΔSij(n-m+1)}/M  (16)

    ΔSij(n)=Sj(n)-Si(n)                                  (17)

FIG. 23 is a more practical embodiment of the system of FIG. 6 accordingto the invention. In the figure, the same parts as those shown in FIGS.6 and 17 are represented with like reference marks and theirexplanations will not be repeated. In FIG. 23, a 3-stage shift register21a of 4-bit width forms a third storage unit 21. The structure of theshift register 21a is the same as that of the shift register 15a. A ROM22a forms a fifth operation unit 22 and has the same structure as theROM 16a. The ROM 22a stores data in advance to convert inputs of 4 bits×4 provided through address lines into an output signal through fourbits of an 8-bit data line.

An operation of this embodiment will be explained. A ROM 11a providesthe shift register 21a with a mean value of four reference signals. Themean value is shifted to the right at each clock pulse in the shiftregister 21a and supplied to one of 4-bit address terminals of the ROM22a. Each of the other three 4-bit address terminals of the ROM 22receives a corresponding one of the three past reference signal meanvalues provided by the shift register 21a in parallel. The ROM 22acalculates a moving average of the mean values at consecutive fourinstants of the reference signal from the present moment to the past,and supplies the moving average to the comparator 12a. Namely, the ROM22a temporally filters the spatially filtered reference signals andprovides the comparator 12a with a result thereof.

The comparator 12a provides an error signal ΔSi(n) according to adifference between the operation result provided by the ROM 22a and anoutput signal of its own node. The error signal ΔSi(n) is expressed asfollows:

    ΔSi(n)= .sub.m=1 Σ.sup.M {.sub.j=1 Σ.sup.N Sj(n-m+1)}/N!/M-Si(n)                                     (18)

This error signal ΔSi(n) is supplied as a control signal to the controlunit 13, which provides the signal Si(n+1) according to the equations(11) and (15).

Similar to the previous embodiment, this embodiment makes the errorsignal ΔSi(n) to be zero when the reference signals are changed, tothereby synchronize the nodes of the network with one another. When anabnormally large or small reference signal is provided, the spatialfiltering operation of the embodiment suppresses a total change due tothe abnormal signal to one fourth, and the temporal filtering operationsuppresses a changing ratio due to the influence of the abnormal signalto one fourth.

The spatial filtering operation may be realized not only by the simpleaveraging but also by the various techniques mentioned before. Also, thetemporal filtering operation may be realized not only by the simplemoving averaging but also by the various techniques mentioned before.Order of the spatial filtering and temporal filtering is not limited tothat of the third embodiment. As shown in FIG. 7, reference signals maybe temporally filtered at first and then spatially filtered.

As shown in FIG. 8, reference signal values may be compared with thesignal quantity of a given node to provide a plurality of error signals,which are then spatially and temporally filtered to provide a controlinput ΔSi(n). In this case, ΔSi(n) is expressed as follows according tothe equation (18):

    ΔSi(n)= .sub.m=1 Σ.sup.M {.sub.j=1 Σ.sup.N Sij(n-m+1)}/N!/M                                          (19)

where,

    ΔSij(n)=Sj(n)-Si(n)                                  (20)

As shown in FIG. 9, the error signals may be spatially filtered at firstand then temporally filtered.

The embodiments mentioned above are applicable for each base radiostation of a mobile communication system, to synchronize clock,frequency, and transmission power of the mobile communication network asa whole. These embodiments are particularly effective for a mobilecommunication network which involves very narrow radio zones and a largenumber of base radio stations.

FIG. 24 is a schematic view showing a more practical embodiment of thenetwork control system of FIG. 10 according to the invention. In thefigure, numeral 2-10 denotes a phase detection unit (corresponding to2-1 of FIG. 10), 2-20 a signal generation unit (corresponding to 2--2 ofFIG. 10), 2-21 a multiplier, 2-22 an adder/subtracter, 2-23 a register,2-24 an oscillator, 2-25 a variable stage shift register that can changethe number of stages between an input and an output according to acontrol signal, 2-30 a control unit (corresponding to 2-3 of FIG. 10),2-31 a comparator, 2-32 an OR gate circuit (0), and 2-33 a variablefrequency divider.

The phase detection unit 2-10 detects a phase difference between a clocksignal Q serving as a controlled value of its own node and a clocksignal R serving as a controlled value from another node, and providesan absolute signal value |E| and a sign S of the phase difference. Thesignal generation unit 2-20 is activated (in response to a timing signalT₃) by the control unit 2-30, to control the phase of the clock signal Qaccording to the phase differential signal |E| and sign S in a way tominimize the phase difference signal |E|. This will be explained in moredetail. The multiplier 2-21 multiplies the phase difference signal |E|and sign S by a predetermined value α (0 to 1) to maintain a loop gainof the system to be less than 1. The adder/subtracter 2-22 adds thephase difference α|E| and sign S to an accumulated phase difference (acontrol signal |C| and sign S) of the register 2-23, thereby updatingthe control signal |C| with sign S. The oscillator 2-24 generates aclock signal Q' having the same frequency as that of the clock signal R.The variable stage shift register 2-25 receives the clock signal Q' andadvances or delays the phase of the output clock signal Q according tothe control signal |C| with sign S.

Under this state, the control unit 2-30 changes intervals of activationof the signal generation unit 2-20 when the system is initialized ordepending on the magnitude of the phase difference signal |E| with signS. For example, when the system is initialized or when the comparator2-31 determines that the phase difference signal |E| is greater than apredetermined value T_(H), the control unit 2-30 reduces the frequencydividing ratio of the variable frequency divider 2-33, to therebyshorten intervals of the timing signal T₃ and promote convergence to thesignal R. In other cases, the control unit 2-30 increases the frequencydividing ratio of the variable frequency divider 2-33, to elongateintervals of the timing signal T₃, thereby removing the influence ofdisturbance, etc.

FIG. 25 is a block diagram of the phase detection unit of the embodimentmentioned above. In the figure, numeral 2-10 denotes the phase detectionunit (corresponding to 2-1 of FIGS. 7 to 9), 2-11 a D-type flip-flop(FF), 2-12 a counter, 2-13 an adder/subtracter, 2-14 a latch, 2-15 aset/reset flip-flop (FF), 2-16 an up/down counter (U/D counter), 2-17 aset/reset flip-flop (FF), D a delay circuit, I an inverter circuit, andN a NAND circuit.

FIG. 26 is a timing chart showing operations of the phase detectionunit. The operations of the phase detection unit will be explained withreference to FIGS. 25 and 26. A clock signal R is provided by anothernode. At each rise of the clock signal R, a series of timing signals T₁and T₂ are generated. A gate signal G from an output of the flip-flop2-11 is reset in response to the timing signal T₁ and set in response toa rise of the last delay circuit D. While the gate signal G is being atlevel HIGH, the counter 2-12 counts up a high-frequency clock signalCLK. This state is indicated as a count signal A.

Set and reset terminals of the flip-flop 2-15 receive the signals R andQ, respectively. Signals Q₁ to Q₃ show typical three phase states of theclock signal Q relative to the clock signal R. The signal Q₁ shows aphase delay, the signal Q₂ an intermediate state, and the signal Q₃ anadvance in a phase b. The signal Q₁ is controlled to be pulled in thedirection of an arrow mark a, and the signal Q₃ is controlled to bepulled in the direction of an arrow mark b. The signal Q₂ may be pulledin any direction.

With respect to the signals Q₁ to Q₃, the flip-flop 2-15 provides theup/down counter 2-16 with up/down control signals U/D_(l) to U/D₃. Theup/down counter 2-16 is activated in response to the gate signal G. Theup/down counter 2-16 counts up the clock signal CLK while the up/downcontrol signal is at level HIGH, and counts down the clock signal CLKwhile the up/down control signal is at level LOW. Accordingly, a countsignal B corresponding to the up/down control signals U/D₁ to U/D₃ forthe up/down counter 2-16 passes through routes (1) to (3). In the routes(1) and (2), the count signal B may change from 0 to -1 at a certainpoint. At this time, the up/down counter 2-16 provides a borrow signalBO, which is stored in the flip-flop 2-17 to provide a sign S for aphase difference signal |E|.

For the routes (1) and (2), the adder/subtracter 2-13 calculates (A+B)and sets a phase difference signal |a|=|A+B| and a sign S=HIGH (delayedphase) in the latch 2-14. For the route (3), the adder/subtracter 2-13calculates (A-B) and sets a differential signal |b|=|A-B| and a signS=LOW (advanced phase) in the latch 2-14.

FIG. 27 is a schematic view showing an embodiment of the network controlsystem of FIG. 11 according to the invention. In the figure, numeral2-40 denotes a temporal filter (corresponding to 2-4 of FIG. 11), 2-41to 2-43 shift registers (Z⁻¹) for sequentially shifting a phasedifference signal |E|, 2-44 a selector for selecting outputs of theshift registers 2-42 and 2-43 or data "0," 2-45 a ROM for providing anadditive mean of address inputs 0 to n, 2-51 a D-type flip-flop (FF),and 2-40' an analog temporal filter.

The phase detection unit 2-10 detects a phase difference between a clocksignal Q of its own node and a clock signal R from another node andprovides an absolute phase difference signal |E| with a sign S. A signalgeneration unit 2-20 controls the phase of the control signal Qaccording to the phase difference signal |E| with sign S in a way tominimize the phase difference signal |E|.

At this time, the temporal filter 2-40' or 2-40 temporally filters(finds out a moving average of) the signal R provided by another node,the phase difference signal |E| with sign S, or a control signal |C|with sign S prepared for the signal generation unit 2-20 from the phasedifference signal. Under this state, the control unit 2-50 changes thefiltering time width of the temporal filter 2-40' or 2-40 when thesystem is initialized or depending on the magnitude of the phasedifference signal |E|.

More precisely, when the system is initialized or when the comparator2-31 determines that the phase difference signal |E| is greater than apredetermined value T_(H), the flip-flop 2-51 is set, and an outputsignal FTW of the flip-flop 2-51 connects input terminals of theselector 2-44 to the B side. As a result, a data width for providing amoving average is narrowed to quicken response of the temporal filter2-40, thereby speeding up convergence to the signal R. In other cases,the flip-flop 2-51 is reset, and the output signal FTW thereof connectsthe input terminals of the selector 2-44 to the A side. As a result, thedata width for providing a moving average is widened to slow theresponse of the temporal filter 2-40, thereby eliminating the influenceof disturbance, etc.

FIG. 28 is a schematic view showing a more practical embodiment of thenetwork control system of FIG. 12 according to the invention. In thefigure, numeral 2-60 denotes an input/output level conversion unit(corresponding to 2-6 of FIG. 12), 2-61 a ROM for storing a plurality oflevel conversion characteristics, 2-70 a control unit, 2-71 a D-typeflip-flop (FF), and 2-60' an analog input/output level conversion unit.

A phase detection unit 2-10 detects a phase difference between a clocksignal Q of its own node and a clock signal R from another node andprovides an absolute signal value |E| with sign S of the phasedifference. A signal generation unit 2-20 controls the clock signal Qaccording to the phase difference signal |E| with sign S in a way toreduce the phase difference signal |E|.

At this moment, the input/output level conversion unit 2-60' or 2-60converts the input/output level of the signal R from another node, ofthe phase difference signal |E| with sign S, or of a control signal |C|with sign S prepared for the signal generation unit 2-20 from the phasedifference signal. The control unit 2-70 changes the level convertingcharacteristics of the input/output level conversion unit 2-60' or 2-60when the system is initialized or depending on the magnitude of thephase difference signal |E|.

More precisely, when the system is initialized or when a comparator 2-31determines that the phase difference signal |E| is greater than apredetermined value T_(H), the flip-flop 2-71 is set, and an outputsignal LC thereof selects one of the characteristics (for example,coarse quantization characteristics, or linear quantizationcharacteristics, etc.,) stored in the ROM 2-61. As a result, aninput/output dynamic quantization range is widened or linearized, topromote convergence to the signal R. In other cases, the flip-flop 2-71is reset, and the output signal LC thereof selects anothercharacteristics (for example, nonlinear quantization characteristicsthat improves quantization accuracy around an input level of 0) storedin the ROM 2-61, thereby improving the quantization accuracy around aninput level of 0 and increasing convergence accuracy.

This embodiment is applicable not only for phase synchronization of aclock signal but also for the magnitude synchronization of varioussignal parameters, control parameters, etc.

FIG. 29 is a block diagram showing an error detection unit of theembodiment mentioned above. In the figure, numeral 2-80 denotes theerror detection unit (corresponding to 2-1 of FIGS. 10 to 12), 2-81 areception unit, 2-82 a demodulation unit, 2-83 an A/D conversion unit,and 2-84 a subtracter.

A signal R provided by another node may be an amplitude modulated signalwith predetermined signal parameters. The reception unit 2-81 receivesthe modulated signal, and the demodulation unit 2-82 demodulates anamplitude among the signal parameters. The A/D converter 2-83 convertsthe demodulated signal into a digital signal. The subtracter 2-84provides a differential signal indicating an amplitude differencebetween the output of the A/D converter 2-83 and a signal parameter P ofits own node, thereby synchronizing the signal parameter, similar to theprevious case.

In this case, the signal generation unit 2-20 may control the amplitudeof the signal parameter P of its own node according to the error signal|E| with sign S in a way to reduce the error signal |E|.

In the above embodiment, the stage variable shift register 2-25 shiftsthe phase of the clock signal Q'. It is possible to directly control theoscillation frequency of the oscillator 2-24 according to the controlsignal |C| with sign S.

In the above embodiment, the comparator 2-31 determines whether or notthe differential signal |E| is greater than a predetermined value T_(H).It is possible to arrange a plurality of threshold values T_(Hi) todetermine the magnitude of the differential signal |E| in multiplesteps, to thereby control a controlled value in multiple steps.

In the above embodiment, only one signal R is provided by another node.It is possible to mutually and simultaneously synchronize a plurality ofnodes with one another. In this case, a plurality of signals R areaveraged, and the averaged signal is provided to the detection unit.Alternatively, differential signals E_(i) between signals R_(i) from aplurality of nodes and a signal Q of a given node are prepared, and thedifferential signals are averaged to provide a differential signal E.

The loop gain α may be changed when the system is initialized ordepending on the magnitude of the differential signal E.

The above embodiment uses the temporal filter 2-40. It is possible toprepare a high pass filter, a notch filter, a band-pass filter, etc., tochange filter characteristics from one to another.

Embodiments of Averaging in Frequency Control (FIGS. 30A to 38)

An embodiment of the spatial averaging (filtering) carried out by theaveraging unit 3-5 of FIG. 13 will be explained. This embodiment employsg_(1i) to g_(6i) for substituting for Δf_(i) (t) in the second term ofthe right side of the equation (7), to control f_(i) (t+1).

(1) The simplest averaging is arithmetic averaging, which will beexpressed as follows according to the equation (5): ##EQU2## whereΔf_(ij) (t) is a relative frequency error between a frequency f_(i) '(t)of an "i"th base station and a frequency f_(j) '(t) of a "j"th basestation adjacent to the "i"th base station and expressed as follows:

    Δf.sub.ij (t)=(f.sub.i '(t)+f.sub.sij)-f.sub.j '(t)  (22)

(2) In the arithmetic averaging of the equation (21), the maximum andminimum values of Δf_(ij) (t) may be excluded from the equation (21) toachieve the following averaging:

    g.sub.2i (t)= .sub.k=1 Σ.sup.N-2 Δf.sub.ik (t)!/(N-2) (23)

where N must be equal to or larger than 3, and k indicates the number"N-2" of base stations that is obtained after excluding the maximum andminimum values from j=1, 2, . . . , N.

(3) In addition to detecting the Δf_(ij) (t), a unit (not shown) fordetecting C/N (carrier power/noise power) or S/N (signal/noise) may bearranged to detect the C/N of each adjacent base station. In this case,each frequency error Δf_(ij) (t) is weighted while C/N=γ_(j), to obtaina weighted mean value:

    g.sub.3i (t)= {.sub.j=1 Σ.sup.N γ.sub.j (t)·Δf.sub.ij (t)}/{.sub.j=1 Σ.sup.N γ.sub.j (t)}!/N                                                   (24)

(4) A combination of the mean values g_(2i) and g_(3i) will provide thefollowing mean value:

    g.sub.4i (t)= {.sub.k=1 Σ.sup.N-2 γ.sub.k (t) Δf.sub.ik (t)}/{.sub.k-1 Σ.sup.N-2 γ.sub.k (t)}!/(N-2)  (25)

(5) The ratio C/N=γ_(j) may have a threshold γ_(th) to exclude Δf_(ij)(t) from averaging when γ_(j) <γ_(th) :

    g.sub.5i (t)={.sub.l=1 Σ.sup.N' Δf.sub.il (t)}/N'(26)

This is used as a mean value. In this case, _(t=1) Σ^(N') means toaccumulate N' pieces of Δf_(il) (t) that satisfy γ₁ >γ_(th).

(6) A combination of the mean values g_(3i) and g_(5i) provides thefollowing mean value:

    g.sub.6i (t)= {.sub.l=1 Σ.sup.N' γl(t) Δf.sub.il (t)}/{.sub.l=1 Σ.sup.N' γl(t)}!/N'            (27)

FIGS. 30A to 30C show an embodiment of a temporal averaging (filtering)operation that is carried out as and when required, after a spatialaveraging operation by the averaging unit 2-5 mentioned above. Thisembodiment employs G^(a) _(ki) (t) (k=1 to 3, a=1 to 6) for substitutingαΔf_(i) (t) of the second term of the right side of the equation (7), tocontrol f_(i) (t+1).

FIG. 30A employs a so-called transversal filter. M pieces of delayelements T receive spatial mean values Δf_(i) (t) of frequency errors inseries. These delay elements T, M pieces of tap coefficients α_(n), andan adder ADD1 provide the following frequency control signal:

    G.sup.a.sub.li (t)=.sub.n=- Σ.sup.M α.sub.n g.sub.ai (t-n) (28)

where α_(n) <1, and a=1 to 6 for indicating the kind of spatialaveraging.

FIG. 30B shows a complete integration filter. In this example,multipliers M₁ and M₂ having multiplication coefficients α and β,respectively adders ADD2 and ADD3, and a delay element T provide thefollowing frequency control signal:

    G.sup.a.sub.2i (t)=αg.sub.ai (t)+βh.sub.ai (t)  (29)

    h.sub.ai (t)=h.sub.ai (t-l)+g.sub.ai (t)                   (30)

FIG. 30C shows an incomplete integration filter. According to thisexample, multipliers M₃ and M₄ having multiplication coefficients α andβ respectively an adder ADD4, and a delay element T provide thefollowing filtered mean value:

    G.sup.a.sub.3i (t)=αg.sub.ai (t)+βG.sup.a.sub.3i (t-l) (31)

This filter corresponds to that of FIG. 30A with the followingconditions:

α_(n) =α

α_(n) =β^(n), n=1, 2, . . . , M

M→∞ (32)

FIGS. 31 to 38 show results of simulations of frequency errors, i.e.,controlled value errors. These simulations have been made with theaveraging unit 2-5 with the filters of FIGS. 30A to 30C carrying outaveraging operations and with the transmission frequency control unit2-6 of FIG. 13 controlling transmission frequencies. In these examples,a base station A is located adjacent to six base stations, as shown inFIG. 14. Base stations B, C, and D, carry out control according tofrequency errors detected with respect to closest two, three, and fourstations, respectively. Under an initial state with t=0, a frequencyerror of each base station occurs at random and is substantiallyuniformly distributed in a range of -1 to +1.

(1) FIG. 31 corresponds to transmission frequency control carried out bythe simple spatial arithmetic averaging of g_(li) (t) of the equation(21). As shown in the figure, frequency errors with respect to theadjacent base stations approach 0 as time elapses. An abscissa indicatestime, and an ordinate indicates normalized frequency errors.

(2) FIG. 32 shows frequency errors according to control based on G^(a)_(li) (t) obtained by a combination of the simple spatial arithmeticaveraging of g_(li) (t) of the equation (21) and the temporal averagingoperation of the transversal filter of FIG. 30A. Here, α_(n) =0.5 andM=8. Compared with those of FIG. 31, frequency errors of FIG. 29 morequickly converge (pull in) and involve a smaller time constant. Thesmaller time constant, however, is vulnerable to noise.

(3) FIG. 33 shows frequency errors obtained by a combination of thesimple spatial arithmetic averaging of g_(li) (t) of the equation (21)and the temporal averaging of the complete integration filter of FIG.30B. Compared with those of FIG. 32, frequency errors of FIG. 33 morespeedily converge. In this example, α=0.8 and β=0.6 in the equation(29).

(4) FIG. 34 shows frequency errors obtained by a combination of thesimple spatial arithmetic averaging of g_(li) (t) of the equation (21)and the temporal averaging of the incomplete integration filter of FIG.30C. This example achieves substantially the same convergence offrequency errors as that of FIG. 33. In this example, α=0.8 and β=0.6 inthe equation (32).

(5) FIG. 35 shows frequency errors obtained by the simple spatialarithmetic averaging operation of g_(li) (t) of the equation (21) with areference station R being set among base stations as shown in FIG. 14.FIG. 35 shows gradual convergence to a transmission frequency of thereference station R. In this example, the frequency error of thereference station R is set to 0.75 to clearly show the effect of thereference station.

(6) FIG. 36 shows frequency errors obtained with use of the referencestation R and a combination of the simple arithmetic averaging of g_(li)(t) of the equation (21) and the temporal averaging of the transversalfilter of FIG. 30A. Compared with FIG. 35, it is understood that thespeed of convergence to the transmission frequency of the referencestation R is faster in FIG. 36.

(7) FIG. 37 shows frequency errors obtained with use of the referencestation R and a combination of the simple arithmetic averaging of g_(li)(t) of the equation (21) and the temporal averaging of the completeintegration filter of FIG. 30B. It is understood that convergence to thetransmission frequency of the reference station R is oscillating.

(8) FIG. 38 shows frequency errors obtained with use of the referencestation R and a combination of the simple arithmetic averaging of theabove item (1) and the temporal averaging of the incomplete integrationfilter of FIG. 30C. It is understood that a waveform of gradualconvergence to the transmission frequency of the reference station Rresembles to that of the transversal filter.

Embodiments of Timing (Phase) Control (FIGS. 39 to 48)

FIG. 39 is a schematic view showing essential part of a more practicalembodiment of the network control system of FIG. 15 according to theinvention. According to this embodiment, a base radio station BS1 isconnected to adjacent base radio stations BS2 to BS5 to form a network,as shown in FIG. 40. FIG. 39 shows essential parts of the base radiostation BS1. Each of the other base radio stations BS2 to BS5 has thesame configuration as the base radio station BS1. A mobile stationcommunicates with any one of the base radio stations BS1 to BS5according to, for example, the TDMA method. At the same time, each ofthe base radio stations BS1 to BS5 transmits a signal having an optionalframe clock frequency and an optional frame phase (controlled values).The embodiment explained below employs the TDMA method. The invention isalso applicable for FDMA or CDMA method, if each of the base radiostations BS1 to BS5 transmits a signal having an optional clockfrequency and an optional clock phase with no regard to frames.

In FIG. 39, comparators 201 to 204 have terminals 206 to 209,respectively. These terminals receive at time t-1, by wire or by radio,information pieces I(f₂, θ₂, t-1) to I(f₅, θ₅, t-1) for frame clockfrequencies f₂ to f₅ and frame phases θ₂ to θ₅ from the base radiostations BS2 to BS5, respectively, as well as an information piece I(f1,θ₁, t-1) of the base radio station BS1 from a voltage control oscillator(VCO) 210. The comparators 201 to 204 compare these signals.

The comparators 201 to 204 provide errors between the frame clockfrequency and frame phase at time t-1 of the base radio station BS1 andthe frame clock frequencies and frame phases at time t-1 of the adjacentbase radio stations BS2 to BS5. In this case, the comparators 201 to 204each compares the numbers of clocks, i.e., clock frequencies thatprovide frames for determining time slots of TDMA with each other andthe clock phases indicating frame heads with each other, and providesthe error information. An adder 211 adds the output signals of thecomparators 201 to 204 to one another, and a divider 212 spatiallyfilters, i.e., divides a result of the addition by the number of theadjacent radio stations (four in total, i.e., BS2 to BS5). A multiplier213 multiplies a result of the spatial filtering by a coefficient αprovided by a coefficient generator 214 and provides a first controlsignal indicating an error ΔB(i, t-1) of the frame clock frequency andframe phase between the base radio station BS1 and the adjacent baseradio stations BS2 to BS5, as shown in the equation (8).

An averaging circuit 216 averages the first control signals and suppliesthe averaged result to a signal processing circuit 215, which convertsthe signal into a second control signal appropriate for controlling theVCO 210. As a result, the VCO 210 is variably controlled to minimize theoutput oscillation frequency and phase error αB(i, t-1). Then, the VCO210 provides the signal B(i, t) of the equation (9) indicating the levelof the node. The output signal of the VCO 210 is transmitted astransmission timing information through wire or radio to each of theadjacent base radio stations BS2 to BS5.

According to this embodiment based on the TDMA method, the frame clockfrequency and frame phase of the base radio station BS1 are corrected tomatch those of the adjacent base radio stations BS2 to BS5. According tothe TDMA method, the clock phase and frame clock frequency are bothcontrolled by the VCO 210. In the FDMA and CDMA methods, the VCO 210controls a clock frequency and a clock phase because these methods areirrelevant to frames.

Next, essential parts of another embodiment of the network controlsystem according to the invention will be explained. This embodimentinvolves base radio stations that communicate with a mobile stationaccording to the TDMA method. In FIG. 39, base radio stations 402 to 404controlled by a radio circuit central control station 401 transmitsignals with the same frame phase and clock frequency as those of otherbase radio stations. A controlled value of this embodiment is only aframe phase.

In FIG. 41, comparators 301 to 304 receive clock phase information I(θ₁,t) of their own base radio station at time t from a phase controlcircuit 309 to be explained later. At the same time, terminals 305 to308 of the comparators 301 to 304 receive clock phase information i(θ₂,t) to I(θ₅, t) at time t of adjacent base radio stations (there are fouradjacent base radio stations in this embodiment) by wire or radio. Thecomparators 301 to 304 detect differences between the information oftheir own station and the information of the adjacent stations. At thistime, the phase control circuit 309 receives reference clock phaseinformation I(θ₀, t) from the central control station 401 of FIG. 42.

An adder 310 adds output signals of the comparators 301 to 304 to oneanother, and a divider 311 divides a result of the addition by thenumber of the adjacent base radio stations (four in this embodiment) andprovides a mean value. A multiplier 312 multiplies the mean value by acoefficient α provided by a coefficient generator 313 and provides afirst control signal indicating a clock phase error between the baseradio station in question and the adjacent base radio stations.

An averaging circuit 315 averages the first control signals and suppliesthe averaged result to a signal processing circuit 314. The signalprocessing circuit 314 converts the received signal into a secondcontrol signal having a proper signal configuration for controlling thephase control circuit 309. The phase control circuit 309 controls thereference clock phase I(θ₀, t) to zero a difference between thereference clock phase I(θ₀, t) and the clock phases of the adjacent baseradio stations. Namely, the phase control circuit 309 receives thereference clock phase I(θ₀, t), adjusts it according to the secondcontrol signal, and provides a clock phase signal of its own station.Output signals of the comparators 301 to 304 provide information relatedto errors between the clock phase of its own station and the clockphases of the adjacent base radio stations. When the FDMA and CDMAmethods are employed, the phase control circuit 309 controls clockphases.

FIG. 43 shows a network based on the above embodiment having 80 nodes.FIGS. 44A to 44c and FIGS. 45A and 45B show results of simulationscarried out on this network with initial values given at random and acoefficient α of 0.5. In each of the figures, an ordinate indicates thelevel B(i, t) of a node, and an abscissa time. FIGS. 44B and 44C showpull-in characteristics of different two nodes. FIG. 44A shows pull-incharacteristics of all of the 80 nodes. As is apparent in FIG. 44A, thelevels of all nodes become equal to one another after a predeterminedperiod.

In FIG. 43, if a node 81 is added to the network, pull-incharacteristics of each node of the network will be slightly disturbedjust after the addition of the node 81 as indicated with "I" in FIG.45A. This disturbance, however, is absorbed to a synchronized state atonce. If a node 73 is removed from the network as shown in FIG. 42,pull-in characteristics of each node of the network will be disturbedjust after the removal of the node 73 as indicated with "II" in FIG.45A. This disturbance is also absorbed to a synchronized state at once.

In FIG. 45B, "a" indicates pull-in characteristics of the added node 80with an initial value of 0, and "b" represents characteristics that thevalue B(i, t) of the node 73 is forcibly zeroed and then removed fromthe network. Even if a node is added to or removed from the network,each node in the network is maintained in a stable synchronized state.

With the TDMA, FDMA, or CDMA method for mobile communication, thisembodiment can synchronize timing of base radio stations with oneanother in order to realize a hand-over with no momentary stoppage. Inthis case, when each node corresponds to a base radio station and whenthe magnitude of the node is given as frequency information or phaseinformation, results of simulations show that the nodes are synchronizedwith one another. According to the embodiment, it is understood thatframe synchronization is not greatly disturbed by addition of base radiostations, or failure of base radio stations.

The invention is not limited to the network arrangement of FIG. 43. Forexample, the invention is applicable for network arrangements of FIGS.46 to 48. In FIGS. 46 and 47, adjacent nodes are basically connected toone another. In FIG. 48, some nodes (base radio stations) are locallysynchronized with one another, and four nodes are disposed around andconnected to each of the locally connected nodes.

Embodiments with Pull-in Range (FIGS. 49 to 52)

When a network is in a mutually synchronized state with a timing phaseserving as a controlled value, and when one node (base station) in thenetwork becomes uncontrollable for the mutual synchronization, thenetwork will converge to a mutual mean value of all nodes because thenetwork has no absolute reference. The effect is that, when one node inthe network becomes uncontrollable, the other nodes follow the value ofthe uncontrollable node, and it will take a long time to restore themutually synchronized state. This phenomenon is shown in FIG. 49, whichis a simulation graph showing characteristics of time versus signalquantity Si(n). As shown in the figure, the mutually synchronized stateis restored anyway but after a long time.

If several nodes become uncontrollable, it will take quite a long timeor be impossible to restore the mutually synchronized state. Thisphenomenon is shown in a simulation graph of FIG. 50. Sincesynchronization never progresses as time elapses, the signal quantitySi(n) never decreases.

FIG. 51 shows means for solving these problems, according to theinvention. In the figure, a signal received by a hybrid 501 isdemodulated by a demodulator 502 and separated by a separator 503 intosignals of individual adjacent nodes. The separated signals are comparedwith a reference value in differential detectors 504-1 to 504-n, whichprovide a pull-in range detector 505 with differences. The received dataare received by a terminal circuit 509 through the separator 503.

When the pull-in range detector 505 detects that any one of thedifferences is exceeding a predetermined threshold value, an indicator506 displays, as alarm information, the node number, etc., of theexceeding difference, and a reset signal generation unit 507 generates areset signal for the node number and provides the reset signal to asynthesis unit 508.

The synthesis unit 508 adds the reset signal to transmission data fromthe terminal circuit 509 and provides it to a delay adjustment circuit510. The delay adjustment circuit 510 delays the transmission data by apredetermined delay time in response to a control signal from thepull-in range detector 505. Thereafter, the transmission data is passedthrough a modulator 504 and transmitted from the hybrid 501.

en the node that caused the exceeding difference receives the resetsignal, the separator 503 and reset signal separator 511 thereofseparate the reset signal and reset the pull-in range detector 505thereof. At the same time, the delay adjustment circuit 510 iscontrolled to adjust pull-in timing into a pull-in range.

As a result, a pull-in time is greatly shortened, as shown in asimulation graph of FIG. 52.

Embodiments of Node Selection (FIGS. 53 to 55)

As explained above, a time for stabilizing the mutual synchronizationcontrol is shortened by setting a pull-in range. When each node selectsits comparison objective nodes at random, a sudden timing differencebetween adjacent nodes may spread adjacent nodes, to thereby taking avery long time (t=140) to stabilize the whole network (FIG. 53).

To solve this problem, FIG. 54 shows a rule for selecting comparisonobjective nodes. The rule is set such that fluctuations in a network donot spread over nodes in the network. Nodes existing in the directionsof arrow marks serve as central nodes of the network, and a given nodeis synchronized only with nodes that are closer to the central node,thereby establishing directionality. In practice, each node is providedwith a directional antenna with which nodes to be selected arepredetermined, thereby establishing directionality of the whole networkas shown in FIG. 51. Control information of each node is providedthrough a broadcast channel such as radio BCCH, and each node canoptionally receive the information and correct an error of its own node.

This technique can shorten (t-27) a convergence time needed forstabilizing the network, as shown in a simulation result of FIG. 55.

Embodiments of Synchronous Control Based on Diversity Reception (FIGS.56 and 57)

As explained above, in the network control system of, for example, FIG.13, the averaging unit 3-5 calculates a spatial mean of a frequencyerror Δf provided by the frequency error detection unit 3-4. At thistime, the Δf is usually weighted by multiplying it by a received C/Nprovided by an S/N detection unit (not shown). This is also achieved inthe timing control of FIG. 12.

In these cases, it is necessary to prepare the S/N detection unit.Instead, this embodiment of the invention employs a diversity receptionmethod and uses, in place of the S/N detection unit, a value measured bya received electric field strength measuring device that is usuallyarranged for each system.

FIG. 56 shows a combination of the network control system of FIG. 13based on frequencies and the diversity reception method. Receptionfrequency automatic control units 3-31 and 3-32 are arranged for signalsreceived by two reception antennas RA1 and RA2, respectively. Measuringdevices (E1, E2) 3-7 and 3-8 measure the received electric fieldstrengths of intermediate frequency signals of the reception frequencyautomatic control units 3-31 and 3-32. A comparison/selection unit 3-9compares values measured by the measuring devices 3-7 and 3-8 with eachother and provides an averaging unit 3-5 with a larger one of themeasured values. The selected result is also sent as a selection controlsignal for received data to a selector 3-10.

FIG. 57 shows a combination of the network control system based ontiming control and the diversity reception. What is different from FIG.56 is that a transmission control unit 3-6 involves a timing controlunit 3-11 for receiving an output signal of an averaging unit 3-5 andcontrolling the timing of a modulator, and a timing error detection unit3-12 for receiving a reception timing signal from one reception controlunit 3-31 as well as a transmission timing signal from the timingcontrol unit 3-11 and providing the averaging unit 3-5 with a timingerror signal ΔT.

In this way, in either of the frequency control and timing control, onehaving a larger received electric field strength is added to a frequencyerror or a timing error in calculating a weighted mean value, therebyomitting the S/N detection unit.

As explained above, the network control system according to theinvention arranges communication means between adjacent nodes. Each nodetransmits its own controlled value. When receiving a controlled valuefrom an adjacent station, a given node spatially or temporally filtersthe received controlled value and controls its own controlled value forthe next time by minimizing a relative error between the controlledvalue that has been spatially or temporally filtered and its owncontrolled value. In this way, the given node communicates data relatedto the controlled values with adjacent nodes and finds a relative errorbetween them. It is possible, therefore, to eliminate the relativeerrors of the controlled values at least around the given node.Consequently, the controlled values are surely corrected in the end overthe whole system.

Even if some of spatially spread reference signals are disturbed orcause abnormality, the spatial filtering operation carried out on errorsignals can remove or relax the influence of the disturbance andabnormality. Even if a string of temporally continuous error signals arepartly disturbed or cause abnormality, the temporal filtering operationcarried out on the error signals can remove or relax the influence ofthe disturbance and abnormality.

According to the invention, a given node controls its own signal in away to minimize a differential signal between the signal of its own andsignals from other nodes according to the differential signal. When thesystem is initialized or depending on the magnitude of the differentialsignal, the invention changes control intervals, filteringcharacteristics, or level conversion characteristics, thereby mostpreferably maintains the speed of convergence of network synchronizationand the stability of the network.

According to the invention, each base station compares its owntransmission frequency with transmission frequencies of adjacent basestations, and controls its own transmission frequency in a way to bringa relative frequency error close to a nominal frequency gap with respectto a reference station. Accordingly, under a steady state, a transientoperation of an AFC for absorbing the frequency error of a mobilestation is carried out only after a power source is turned ON.Similarly, it is not necessary for a base station to transiently operatethe AFC when a mobile station enters into the zone of the base stationin question or newly transmits a signal. This may greatly reducemomentary communication stoppage during a hand-over.

The invention correctly controls a frequency gap between radio channelsof adjacent base stations. Accordingly, it is not necessary to provide amargin in the frequency gap between the radio channels of the adjacentbase stations in consideration of a frequency error, thereby improvingthe efficiency of frequency use in the communication system as a whole.

The invention can form a distributed control network according to anyone of TDMA, FDMA, and CDMA methods, with base radio stations serving asnodes. Unlike a conventional centralized control network, the inventioncan increase the number of nodes without substantially changing load oneach node. According to the invention, each base radio station isconnected to a plurality of adjacent base radio stations, so that, evenif a given base radio station is disconnected from any one of itsadjacent base radio stations, the given base radio station is able tostably carry out synchronous control according to the remaining normalbase radio stations, thereby providing a backup function.

We claim:
 1. A network control system for controlling a plurality ofnodes respectively corresponding to, and provided in, a plurality ofradio zones, each of the plurality of nodes having a plurality ofcorresponding adjacent nodes without other nodes positioned between therespective node and each of the plurality of corresponding adjacentnodes, each node having a corresponding base station, where a mobilstation travels among the plurality of radio zones and communicates withthe base station of a respective node when travelling in a radio zonecorresponding to the respective node, each base station transmitting atransmission signal for the corresponding node to the plurality ofcorresponding adjacent nodes and receiving transmission signalstransmitted from the base stations of the plurality of correspondingadjacent nodes, each of the base stations comprising:a receptionautomatic frequency control unit receiving the transmission signal ofeach of the base stations respectfully corresponding to the plurality ofcorresponding adjacent nodes, performing an automatic frequency controloperation for each received transmission signal in accordance with thetransmission frequency of the respective received transmission signal,and producing a corresponding frequency signal for each receivedtransmission signal; a frequency error detection unit receiving thefrequency signals produced by the reception automatic frequency controlunit, detecting a frequency error between the frequency of eachfrequency signal produced by the reception automatic frequency controlunit and a frequency obtained by adding a nominal frequency gap betweenbase stations to the transmission frequency of the transmission signalof the node corresponding to the frequency error detection unit, andproducing corresponding output signals; an averaging unit receiving theoutput signals of the frequency error detection unit, spatiallyfiltering the frequency errors, and providing an output signalindicating a spatial mean value corresponding to the spatially filteredfrequency errors; and a transmission frequency control unit receivingthe output signal of the averaging unit and controlling the transmissionfrequency of the node corresponding to the transmission frequencycontrol unit in accordance with the spatial mean value, to substantiallyeliminate the frequency errors.
 2. A network control system according toclaim 1, wherein the averaging unit of each base station averages thefrequency errors with respect to time.
 3. A network control systemaccording to claim 1, wherein at least one base station is a referencestation for transmitting an absolute transmission frequency.
 4. Anetwork control system according to claim 2, wherein at least one ofbase station is a reference station for transmitting an absolutetransmission frequency.
 5. A network control system according to claim1, wherein the mobile station communicates with the base stations viasignals transmitted from the mobile station to the base stations and viasignals transmitted from the base stations to the mobile station, thetransmission frequency of the signals transmitted from the mobil stationto the base stations being a different frequency than the transmissionfrequency of signals transmitted from the base stations to the mobilstation, and each of the base stations and the mobile station controls afrequency difference between the transmission frequency of signalstransmitted from the mobile station to the base stations and thetransmission frequency of signals transmitted from the base stations tothe mobile station.
 6. A network control system according to claim 1,wherein each node has a pull-in range, and the base stations eliminatefrequency errors only on received transmission signals that fall in thepull-in range.
 7. A network control system according to claim 1, whereinthe mobile station communicates with the base stations via signalstransmitted from the mobile station to the base station and via signalstransmitted from the base stations to the mobile station, and arespective node for providing the transmission frequency of signalstransmitted from the base stations to the mobile station is selectedaccording to a certain rule.
 8. A network control system according claim1, wherein the averaging unit of each base station spatial filters thefrequency errors via a weighted mean of relative errors according to areceived electric field strength selected by a diversity receptionmethod.